Anaerobic compositions are mixtures of polymerizable acrylate ester monomers, and peroxy polymerization initiators therefor, the cure of which is inhibited by oxygen. These materials remain in the uncured state as long as adequate contact is maintained with atmospheric oxygen, such as in a partially empty polyethylene bottle. When, however, the composition is placed between metal or other impervious surfaces, contact with oxygen is prohibited and within a relatively short period of time the polymerization reaction commences. These compositions have found their greatest utility as adhesives and sealants (hereinafter frequently referred to jointly as adhesives) since, unlike prior adhesives, cure does not depend upon evaporation of a volatile solvent or the mixing of two reactive components at the time of use.
The earliest disclosed commercially acceptable anaerobic compositions are those in U.S. Pat. No. 2,895,950 to Krieble, issued July 21, 1959. Other typical disclosures of related or improved anaerobic compositions may be found, for example, in U.S. Pat. Nos. 3,043,820 to Krieble, issued July 10, 1962; 3,046,262 to Krieble, issued July 24, 1962; 3,218,305 to Krieble, issued Nov. 16, 1965; and 3,435,012 to Nordlander, issued Mar. 25, 1969.
While the one-component anaerobic adhesives and sealants have gained wide acceptance in commercial practice, they have not realized their full potential because of what heretofore have been certain inherent shortcomings. They are precatalyzed (i.e., a peroxy initiator is included as an inherent part of the composition), and in nearly all cases contain latent accelerators of free radical polymerization. These latent accelerators are substances which, while not initiating polymerization, do increase the rate of polymerization once it has been started by the peroxy initiator. Such ingredients, in combination with other influences which were not known or clearly understood, created an erratic and recondite danger of spurious polymerization in the bottle or elsewhere prior to the time of intended use. The problems were particularly acute when dealing with compositions containing the more active latent accelerators of polymerization.
Attempts to overcome this problem have followed two common paths. First, polymerization inhibitors, such as quinone type inhibitors, have been used in conjunction with the anaerobic compositions. Wyile an aid in the reduction of spurious polymerization, this approach has not eliminated the problem. Moreover, when more sophisticated latent accelerators were discovered (such as those which will be discussed more fully hereinafter), it was found that the inhibitors did adversely affect the speed of cure of the composition in use. Secondly, in order to provide a suitably stable product, less active latent accelerators commonly were used, thus removing from the anaerobic compositions some of the potential speed of cure which could be obtained otherwise.
It recently has been hypothesized that a significant portion of the unpredictability inherent in anaerobic compositions is tracable to the presence of metal contamination inherent in the starting materials, or acquired during processing. Based on this hypothesis, attempts were made to eliminate the problem, such as by greater care in the manufacturing operations for the anaerobic composition and starting materials therefor, and attempts to reduce the metal content in such products. While some of the processes did provide certain benefits, none was found to be an acceptable solution. It now appears that, in fact, levels of metal contamination substantially below that which can be removed by conventional processing are more than enough to cause the serious stability and spurious polymerization problems previously noted, particularly anaerobic compositions which contain active latent polymerization accelerators. In some systems, metal levels substantially below one part per million are still significant. Thus, the depths of this problem are substantially greater than has been thought previously. Hence, the reason for the lack of success to date in finding an adequate solution.
Among other things, this invention is designed to provide anaerobic compositions and processes for preparing them which materially reduce or substantially eliminate the above-described problems of the prior art, greatly simplify the manufacture of anaerobic compositions, and make available in such processes and compositions the use of higher levels of active polymerization accelerators than has been commercially possible heretofore.